As a multiple carrier transmission which utilizes the orthogonal frequencies, OFDM (orthogonal Frequency Division Multiplex) can efficiently take advantage of frequencies and has better performance in mitigating multipath distortion. Since the OFDM modulated wave has a high peak factor, if the back-off of the amplifier is not sufficient, the transmission quality will be degraded due to the nonlinear distortion of the amplifier or the disturbance to adjacent channels will be increased due to the elevated sidelobe level of the frequency spectrum. Further, in an operational amplifier of class A or class AB, the efficiency of the amplifier decreases as the back-off increases. Thus, an amplifying process is expected that the efficiency is not decreased even at the moment when the back-off is obtained. The envelope tracking is known as one of such amplifying processes. For example, JP2010-206545 has proposed a scheme in which the envelope of the input signals is detected and the bias voltage of the amplifier is changed depending on whether the level of the envelope has gone beyond the threshold. Besides, WO02/003545 has proposed a scheme in which the bias current of the amplifier is adjusted by a current mirror circuit according to the variation of the level of the envelope of input signal.